step1：使用driver_id相等的条件，将online_time_tb左连接driver_tb；

step2：使用city_id相等的条件，左连接city_tb;

step3：限定条件为北京、8月和9月的新司机；

step4：按月份分组，求在线总时长。

SELECT DATE_FORMAT(a.dt,'%Y-%m') AS month, SUM(online_len) as 'online_len'

FROM online_time_tb a

LEFT JOIN driver_tb b using(driver_id)

LEFT JOIN city_tb c using(city_id)

WHERE c.city_name='北京' and ((DATE_FORMAT(b.finish_dt, '%Y-%m')='2021-08'

AND DATE_FORMAT(a.dt, '%Y-%m')='2021-08')

OR (DATE_FORMAT(b.finish_dt, '%Y-%m')='2021-09'

AND DATE_FORMAT(a.dt, '%Y-%m')='2021-09'))

GROUP BY DATE_FORMAT(a.dt,'%Y-%m');